> I don't understand why it took LineageOS so long to update to AOSP 17, while GrapheneOS managed to update to 17 in just 3 days.

Here's a hint: GrapheneOS has paid developers working on it full-time, while LineageOS is done by people in their spare time. Also, GrapheneOS has a collaboration with Motorola and through that gets for instance early access to security patches, and probably other things as well. And lastly, LineageOS supports roughly 10x the number of devices. It's significantly easier if you restrict yourself to Pixels.